1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to a method for improving the communication success rate in a simultaneous call trial between subscribers.
2. Description of the Related Art
Generally, a base station, a Mobile Switching Center (MSC), a wire cable, and electric waves are used for communication between two counterparts by a mobile communication terminal. A mobile communication terminal connects with a base station through electric waves, with the base stations connected through optical fibers , satellites, etc. Further, the MSC is disposed between base stations and performs various functions to support wireless communication. In addition, a mobile communication terminal frequently registers its own location in the MSC of a corresponding base station. When a call is received, the mobile communication terminal connects with the corresponding base station according to the registration and performs communication.
A core network for operation of a mobile communication terminal includes an MSC, a Home Location Register (HLR), a Visitor Location Register (VLR), a Service Control Point (SCP), and an Intelligent Network (IN).
The MSC processes signals originated from, or terminated in, each mobile base station and performs a central control function for controlling each mobile base station to operate efficiently. Further, the MSC tracks the mobile communication terminal location and asks the HLR for location registration information of a user. The HLR informs the MSC of various information relating to an originating state or a terminating state of a subscriber, a supplementary service, etc., and a subscriber location. When the location information is received from the HLR, the MSC simultaneously calls mobile communication terminals of users through all base stations located in a corresponding area.
The SCP is a database storing information for controlling a service provided to a subscriber. A representative example of the SCP is the HLR. Further, the SCP relates to the multiple subscriber number service (e.g., 800 numbers), a caller ID service, a short message service, etc.
The HLR is a database for storing subscriber information and includes service type and home switch office ID information. A home location is a location first searched for determining a location of a mobile communication terminal. Generally, a base station nearest to the place at which a subscriber has joined the service is registered as a home location. When a user calls for the first time, an MSC within the base station controlled by a corresponding switching center tracks a home switch office identifier (ID) registered in the HLR and checks whether the terminating terminal exists in the region controlled by the base station. If the corresponding terminating terminal does not exist in the region, the MSC searches a database of the VLR.
The VLR is a database for registering a location of a terminal deviating from a home switch office. A terminal frequently reports its own location through a nearest base station and the ID of the base station is registered in the VLR. When a terminating terminal is not found within the home location after a user telephones, the MSC finds out the ID of the base station registered in the VLR and sends a signal to the found base station. In this way, the user connects with the terminating terminal.
Herein, a ‘Send Routing Information (SRI)’ message is a message transmitted from the MSC to the HLR to obtain a VLR address of a terminating network where a receiver is located. The HLR sends a ‘Provide Roaming Number (PRN)’ message to the network of the called subscriber after receiving the ‘SRI’ message. The PRN message is used for requesting a roaming number of a terminating VLR, that is, an address value.
Further, an Initial Address Message (IAM) is a standard interoffice signaling message of an Integrated Service Digital Network (ISDN), which is transmitted when an originating MSC first tries signaling to a terminating MSC. The terminating MSC transmits an Address Complete Message (ACM) to the initial MSC in response to the IAM. Further, the terminating MSC may receive supplementary service information of the caller by checking a receiver number of a caller, that is, a Mobile Station International Subscriber Directory Number (MS-ISDN) through the IAM. Further, the terminating MSC may allocate necessary resources through the IAM.
In the mobile communication system as described above, when subscribers A and B simultaneously try voice calls to each other, both lines of the subscribers A and B are busy. This is because the subscribers A and B have already started a signaling to communicate with each other and cannot process incoming calls.
FIGS. 1A to 1C are block diagrams illustrating cases in which subscribers simultaneously try calls to each other in a conventional mobile communication system.
FIG. 1A shows a case in which an MSC/VLR 100 of a subscriber A and an MSC/VLR 150 of a subscriber B simultaneously try routing to find roaming numbers of counterparts. In such a case, the subscribers A and B go into a busy state due to collision between messages thereof and will both get busy tones.
FIG 1B shows a case in which an MSC/VLR 100 of a subscriber A performs routing to find a roaming number of an MSC/VLR 150 of a counterpart B. In such a case, the MSC/VLR 150 of the counterpart B transmits an ‘IAM’ message to the MSC/VLR 100 of the subscriber A after the routing, so that an outgoing call is being processed.
FIG 1C shows a case in which an MSC/VLR 100 of a subscriber A has already performed a routing or an outgoing call is progressing, and an MSC/VLR 150 of a subscriber B is just performing an originating call signaling.
In the cases shown in FIGS. 1A to 1C, a call trial ends in a failure (reason: busy_subscriber) and the subscribers A and B both get busy tones.
In the conventional mobile communication system described above, when subscribers simultaneously try calls to each other, the call trial ends in a failure and the subscribers both get busy tones. Since the simultaneous call trial basically aims at communication with the counterpart, the busy tones are to be expected. In such a case, each subscriber may recognize that the counterpart is communicating with another person, and may call again after a while or give up the communication. As a result, it may become impossible to establish the communication between the subscribers.